With the Gargoyle Glk Extensions, there is now no need to change to Z-Code in order to specific per-character colours. Interpreter support is gradually being added. Right now Gargoyle, Windows Glk, and Spatterlight support the extensions, and soon Parchment and hopefully Lectrote will.
If you don’t need per-character colours, then you can just use Glulx Text Effects to set up colours and other formatting options.